Question

Is a hemiacetal a reducing sugar?

Answer 1

Hint: Hemiacetals are obtained as a result of the addition of an alcohol to an aldehyde and most of the sugars are hemiacetals. The most commonly known hemiacetal is the sugar glucose. The favorability of the formation of a strain free $6$ membered ring and the electrophilicity of an aldehyde combine to strongly favor the acetal form.

Complete answer:
A hemiacetal is carbon connected to two oxygen atoms where one oxygen is an ether and the other oxygen is an alcohol. A reducing sugar is any reducing sugar that is capable of acting as a reducing agent because it has a free ketone group or a free aldehyde group. In order for the oxidation to take place, the cyclic form must first open to give reactive aldehyde. So, a sugar which contains a hemiacetal will be a reducing sugar. Sugars with a hemiacetal are in equilibrium with a ring-opened form containing an aldehyde and they will react as a reducing agent towards some metal salts. Note that a sugar without a hemiacetal is a non-reducing sugar. Acetals are locked and are not in equilibrium with a ring-open form with an aldehyde. Therefore, they will not react as a reducing agent.
Therefore, a hemiacetal is a reducing sugar.

Note:
 Hemiacetal is formed when an aldehyde reacts with an alcohol. There are two ways in which this formation can occur: as a neutral reaction or catalyzed with an acid. The neutral reaction involves only the alcohol and aldehyde whereas the acid catalyzed reaction puts a hydrogen on the aldehyde oxygen to start with.